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Enamel Structure in Astrapotheres and Its Functional Implications

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Enamel Structure in Astrapotheres and Its Functional Implications Scanning Microscopy Volume 6 Number 2 Article 15 6-24-1992 Enamel Structure in Astrapotheres and Its Functional Implications John M. Rensberger University of Washington Hans Ulrich Pfretzschner University of Bonn Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Biology Commons Recommended Citation Rensberger, John M. and Pfretzschner, Hans Ulrich (1992) "Enamel Structure in Astrapotheres and Its Functional Implications," Scanning Microscopy: Vol. 6 : No. 2 , Article 15. Available at: https://digitalcommons.usu.edu/microscopy/vol6/iss2/15 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 6, No. 2, 1992 (Pages 495-510) 0891- 7035/92$5. 00 +. 00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA ENAMEL STRUCTURE IN ASTRAPOTHERES AND ITS FUNCTIONAL IMPLICATIONS John M. Rensberger* 1 and Hans Ulrich Pfretzschner 2 1Department of Geological Sciences and Burke Memorial Washington State Museum, University of Washington, Seattle; 2Institut fiir Palaontologie, University of Bonn, Bonn, Germany. (Received for publication February 17, 1992, and in revised form June 24, 1992) Abstract Introduction Astrapotheres, large extinct ungulates of South Mammalian teeth have a great range of form and these America, share with rhinoceroses vertical prism decussation differences have been used extensively to determine phyletic in the cheek tooth enamel. The similarity extends beyond relationships and evolutionary histories. The dental diversity in merely the direction of the planes of decussation. The mammals derives in part from the diversity of dental function vertical decussation in astrapotheres is confined to the inner and behavioral use of teeth, which include food gathering and part of the enamel and has uniformly well-defined zones in processing, offensive and defensive behaviors as well as social which the prism direction differs by nearly 90° and the zones activities (Crompton & Hiiemae, 1969). are separated by narrow transitional borders of intermediate It is well known that the microstructure of mammalian prism direction. The outer enamel consists of predominantly tooth enamel varies considerably among taxa (e.g. occlusally and outwardly directed prisms. Within the outer Korvenkontio, 1934, Kawai, 1955; Boyde, 1978). Recently, enamel is a region of horizontally decussating prisms; here evidence has been found that differences in enamel the angle of decussation is usually smaller than that of the microstructure in at least some taxa have not arisen randomly inner vertically decussating prisms. but somehow represent responses to selective factors in the Except for the horizontal decussation in the outer chewing mechanics and dietary adaptations. A directional enamel, these conditions match structures that have been asymmetry of the enamel microstructure within single molars described for rhinocerotoids. These features, together with of arvicoline rodents is related to the direction of motion of the the similarity in premortem crack direction and gross shape jaws during mastication (von Koenigswald, 1980). A of the cheek teeth, imply that astrapotheres and reorientation of the microstructural fabric in tapiroids and rhinocerotoids shared essentially the same system of cheek rhinocerotoids, in which the occlusal surface has become tooth mechanics. highly lophodont, is related to the attitude of the functional However, the microstructure of the canine enamel in edges of the lophs and hence to the masticatory mechanism the astrapotheres is distinct. The lower canine enamel of the (Rensberger & von Koenigswald, 1980; Fortelius, 1985; Oligocene Parastrapotherium exhibits a form of vertical Boyde and Fortelius, 1986). Prism decussation is weak or decussation modified by a wavelike bending of prism zones, absent in the soft food eating ceboid primate Alouatta but well whereas the decussation in the rhinocerotoid canine is developed in Cebus apella, C. albifrons, and Chiropotes, horizontal. The lower canine in Parastrapotherium was whose diets include hard objects (Maas, 1986). The directions subjected to different loading conditions, judging from of the prisms in the molars of the koala and in the opossum multiple sets of premortem crack directions. The modified are related to the direction of the occlusal force (Young, et al., vertical decussation would in theory resist cracking under 1987; Stem, et al.,1989). different directions of tensile stresses. This is confirmed by The ubiquity of enamel in the teeth of vertebrates from the sinuous paths of cracks that run in directions differing by fishes to mammals, its hardness, and its arrangement as a thin up to 90°. That diverse stresses were generated in the enamel outer layer together leave little doubt that its fundamental during life is confirmed by the pattern of premortem cracks in function is to resist abrasive wear. Enamel is anisotropic in its Parastrapotherium. The enamel in the upper canine of a late response to abrasive wear (Rensberger and von Koenigswald, Miocene astrapothere lacks decussation but may have 1980) which derives from the fact that apatite crystals abrade resisted cracking under varied loading conditions by virtue of less quickly in a direction parallel to the C axis than in a a 3-dimensional wavelike bending of the prisms. direction normal to the C axis. In the outer enamel, the C axes of the crystallites tend to be directed toward the outer surface, Key Words: Enamel microstructure, molar, canine, which minimizes the rate of wear in a direction normal to the stresses, crack patterns, fracture resistance, functional morph­ surface. ology, convergent evolution, Astrapotheria, Astrapotherium, The major structural limitation of materials with Parastrapotherium, Rhinocerotoidea. ceramic-like properties is their brittleness (Clegg, et al., 1990). The brittleness of enamel makes it subject to cracking and thus * Address for correspondence: to initiating failure of the tooth. The tendency to crack limits Department of Geological Sciences the degree of stress concentration that can be developed in and Burke Memorial Washington State Museum teeth, which in tum limits the toughness and hardness of food University of Washington DB-10 materials that can be physically reduced to small particles by the Seattle, Washington 98195 masticatory system and the speed with which large volumes Phone No.: (206) 543-7036; FAX: (206) 543-9285 can be processed. A conspicuous correlate of the early 495 J. M. Rensberger & H. U. Pfretzschner Cenozoic diversification of mammals was the increase in body the enamel microstructure of astrapothere cheek teeth with that size, which raised the maximum level of stress attainable at the in the rhinocerotoids, describe the microstructure in occlusal surface through stress concentrating mechanisms. astrapothere canine enamel, which differs from that in the Correlating with larger body sizes was the presence of a new cheek teeth, and consider the functional implications of these microstructure that improved the crack resistance of enamel conditions for tooth use. (von Koenigswald, et al., 1987) and allowed higher stresses to Institutional Abbreviations be attained before fracture. High stress concentrations FMNH: Field Museum of Natural History, Chicago, resulting in tooth loss occur as an accidental and probably Illinois. frequent event in lower vertebrates, but the evolutionary GPIBO: Institut fiir Palaontologie der Universitat, solution of more rapid tooth replacement was not available to Bonn, Germany diphyodont mammals, in which the masticatory system is UCMP: University of California Museum of based on precise occlusion (Hopson & Crompton, 1969; Paleontology, Berkeley, California. Crompton & Sun, 1985). UWBM: Burke Memorial Washington State Museum, Thus, owing to high stresses developed in their teeth and University of Washington, Seattle, the diphyodont tooth replacement scheme, enamel in Cenozoic Washington mammals of medium to large size must have been subject to selection for both crack resistance and abrasion resistance. Methods Fracturing is still a limiting factor in modern Carnivora that heavily load their teeth in the course of eating bones (Van The scanning electron microscope (SEM) examinations Valkenburgh & Ruff, 1987; Van Valkenburgh, 1988). of astrapothere enamel microstructure are based on tooth Because both enamel and dentin are denser than other fragments of Parastrapotherium (Deseadan, early Oligocene of hard tissues, teeth are the best preserved structures in the Pico Truncado, Santa Cruz, Argentina), Astrapotherium sensu ma."Illilalian fossil record. Because of this large resource and lato (Friasian, late Miocene La Venta Fauna, Colombia, S.A.), its potential information about dietary behaviors, a number of and Astrapotherium sp. (Santacrucian, early Miocene, workers have attempted to interpret fossil diets by investigating Argentina) that are too incomplete to provide traditional the relationships between striae, pits, and other features of quantitative data about gross tooth morphology and systematic abrasive wear preserved on both fossil and modern tooth relationships. Similar fragments of Subhyracodon (Chadron enamel (Teaford, 1988, reviews this literature). As our Formation, ~arly Oligocene, South Dakota), a rhinocerotoid,
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